Semiconductor electrodes, design requirements

At the same time, SWNT can be used at the top electrode (that needs to be transparent) as a replacement for the currently used thin films that are also electrical conductors. Typical conductive thin films used today are oxides such as fluorine-doped tin oxide (FTO), Al-doped zinc oxide, and the indium tin oxide (ITO) mentioned above. The disadvantages of these oxide films are that they require expensive deposition procedures at high vacuum, they have poor mechanical properties, and they are not transparent in the infrared. Another interesting difference is that transparent oxide conductors are n-type semiconductors. By contrast, nanotube networks act as p-type semiconductors, which could lead to new designs. 

What factors determine the activity of electrocatalysts and are likely to introduce specificity into an electrode reaction At the present time, a complete answer to this question is not possible and we have certainly not reached the stage where it is possible to design electrocatalysts from theoretical considerations. On the other hand a number of general principles can be set out. Thus while metals, alloys, semiconductors (particularly oxides) and complexes have been shown to exhibit catalytic properties, invariably catalysts are based on transition metals and it seems that the design of a catalyst requires the placing of transition-metal ions or atoms in a matrix which serves to optimize their electronic configuration and position with respect to each other. 

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